1. Field of the Invention
The present invention relates generally to lithography systems. More particularly, the present invention relates to management of actinic heat load on mirrors in lithography systems.
2. Background Art
Lithography is a process used to create features on the surface of substrates. Such substrates can include those used in the manufacture of flat panel displays, circuit boards, various integrated circuits, and the like. A frequently used substrate for such applications is a semiconductor wafer. One skilled in the relevant art would recognize that the description herein would also apply to other types of substrates.
During lithography, a wafer, which is disposed on a wafer stage, is exposed to an image projected onto the surface of the wafer by an exposure system located within a lithography system. The exposure system includes a reticle (also called a mask) for projecting the image onto the wafer.
The reticle is generally located between a semiconductor chip and a light source. In photolithography, the reticle is used as a photo mask for printing a circuit on a semiconductor chip, for example. Lithography light shines through the mask and then through a series of optical lenses that shrink the image. This small image is then projected onto the silicon or semiconductor wafer. The process is similar to how a camera bends light to form an image on film. The light plays an integral role in the lithographic process. For example, in the manufacture of microprocessors (also known as computer chips), the key to creating more powerful microprocessors is the size of the light's wavelength. The shorter the wavelength, the more transistors can be etched onto the silicon wafer. A silicon wafer with many transistors results in a more powerful, faster microprocessor.
As chip manufacturers have been able to use shorter wavelengths of light, they have encountered a problem of the shorter wavelength light becoming absorbed by the glass lenses that are intended to focus the light. Due to the absorption of the shorter wavelength light, the light fails to reach the silicon wafer. As a result, no circuit pattern is created on the silicon wafer. In an attempt to overcome this problem, chip manufacturers developed a lithography process known as Extreme Ultraviolet Lithography (EUVL). In this process, a glass lens can be replaced by a mirror. Although the mirror reflects a large percentage of the light, a fair amount of the light is absorbed by the mirror. The absorbed actinic light (i.e., energy generated from a light source such as an optical light source in a lithography tool) causes heat load on the mirror. Too much heat can result in image distortion on the wafer. Further, if heat load on the mirror is not maintained at a relatively constant level, variation in the amount of image distortion can occur. Thus, there is a need to control actinic heat load (e.g., by measuring mirror temperature) on the mirror caused by the absorbed light.
The temperature of the mirror should be controlled such that the temperature is maintained constant over time. Conventional mirror temperature control techniques attempt to maintain a time-constant mirror temperature by varying the rate of heat removal from the non-optical surfaces of the mirror with a temperature servo. A typical mirror is relatively large and has a high thermal mass with low thermal conductivity. Due to the two above mentioned characteristics of the typical mirror in a lithography projection system, this conventional “control-by-heat-removal” method can be ineffective in environments with transient actinic heat loads. For example, in applications such as EUV photolithography of integrated circuits, the actinic heat load is transient (e.g., changes every time a reticle is exchanged). The actinic heat load changes faster than the temperature control servo's ability to follow. As a result, the temperature of the mirror is not maintained at a constant over time and variation in distortion of the projected image occurs.
The problem of image distortion variation resulting from failure to maintain a time-constant and spatially-constant heat load on the mirror is further exacerbated by a phenomenon known as “the cold edge effect.” The cold edge effect is caused by the variation of actinic heat load on the optical aperture of the mirror and the annular area (i.e., the non-illuminated area of the mirror located beyond the optical aperture). A lithography mirror typically has a lower temperature at the annular area than it has at the optical aperture.
Therefore, what is needed is an apparatus and method for fabricating a mirror and for managing heat load on the mirror such that variation in image distortion from variation of heat on the mirror is minimized. Such an apparatus and method should maintain a time-constant total heat load during transients of illumination incident on the projection mirror (i.e., during times of change of actinic heat load on the mirror). Further, such an apparatus and method should also maintain a spatially constant total heat load on the mirror to mitigate the cold edge effect.